Filters are known to provide attenuation of signals having frequencies outside of a particular frequency range and little attenuation to signals having frequencies within the particular frequency range of interest. As is also known, these filters may be fabricated from ceramic materials having one or more resonators formed therein. A ceramic filter may be constructed to provide a lowpass filter, bandpass filter or a highpass filter, for example.
For bandpass filters, the bandpass area is centered at a particular frequency and has a relatively narrow bandpass region, where little attenuation is applied to the signals. While this type of bandpass filter may work well in some applications, it may not work well when a wider bandpass region is needed or special circumstances or characteristics are required.
Block filters typically use an electroded pattern on an outer (top) surface of the ungrounded end of a combline design. This pattern serves to load and shorten resonators of a combline filter. The pattern helps define coupling between resonators, and can define frequencies of transmission zeros.
These top metallization patterns are typically screen printed on the ceramic block. Many block filters include chamfered resonator through-hole designs to facilitate this process by having the loading and coupling capacitances defined within the block itself, for manufacturing purposes. The top chamfers help define the intercell couplings and likewise define the location of the transmission zero in the filter response. This type of design typically gives a response with a low side zero. To achieve a high side transmission zero response, chamfer through-holes are placed in the grounded end (bottom) of ceramic block filters, for example. Thus, a high zero response ceramic filter would typically have chamfers at both ends of the dielectric block. A double chamfer filter can be difficult to manufacture because of the tooling requirements and precise tolerances.
A filter which can be easily manufactured to manipulate and adjust the frequency response, preferably with a frequency adjustable shunt zero, to attenuate unwanted signals, could improve the performance of a filter and would be considered an improvement in filters, and particularly ceramic filters.
In duplexed telecommunications equipment, such as cellular telephones, two frequency ranges are normally allocated, one for transmitting and one for receiving. Each of these frequency ranges is subdivided into many smaller frequency ranges known as channels, as shown in FIG. 1. Bandpass filters in this equipment should be made to pass (with minimal attenuation) the entire transmit or receive frequency range, and attenuate the entire receive or transmit frequency range, respectively, even though the device will be using only one channel in each range at any given time. These filters must necessarily be larger than a filter with an equivalent performance, which operates over only a few channels.
A bandwidth of a filter can be designed for specific passband requirements. Typically, the tighter the passband, the lower the insertion loss, which is an important electrical parameter. However, a wider bandwidth reduces the filter's ability to attenuate unwanted frequencies, typically referred to as the rejection frequencies. The addition of a shunt zero in the transfer function at the frequency of the unwanted signal, could effectively improve the performance of a filter, as detailed below.
A mass-producable, dynamically tunable (or adjustable) filter which can modify the frequency response by attenuating unwanted signals, could improve the desired performance of a filter and would be considered an improvement in filters.